Vinyl aromatic compounds such as styrene undergo undesired spontaneous polymerization (i.e. polymerization of monomers due to heat or the random generation of free radicals in the monomers) during storage, shipping or processing. The problem is particularly acute during purification operations carried out at elevated temperatures, such as distillation. Spontaneous polymerization is disadvantageous not only because it causes fouling of distillation column reboilers and other equipment used for processing vinyl aromatic monomers but also because it usually renders the monomers unfit for use without further treatment. Accordingly, it is desirable and often necessary to inhibit the spontaneous polymerization of vinyl aromatic monomers.